1. Technical Field
The present invention generally relates to imaging systems. More particularly, the present invention relates to imaging systems utilizing photolithographic techniques.
2. Background Art
Optical projection systems are used in many scientific and industrial applications, an example of the latter being the manufacture of microcircuits on a semiconductor substrate or wafer, the process being referred to as "photolithography". In lithography, mask patterns, typically opaque chrome on glass, are imaged onto a semiconductor wafer coated with a light-sensitive material often referred to as "photoresist" or "resist". The wafer is then subjected to several processing steps, including developing the photoresist and etching the wafer to create various circuit structures. To achieve successful microcircuit performance, the imaging system must faithfully reproduce the mask pattern in the photoresist. Due to ever-decreasing size constraints, many of these imaging systems are often operated near or at their resolution limit. Even with a projection system that is optically perfect, variations in the size and shape of the imaged features may still occur, due to, for example, nonuniform mask illumination or optical aberrations, such as field curvature, coma, astigmatism and spherical aberration. These occurrences are capable of significantly altering the image of the object (i.e., feature size or line width) over the imaging plane. It is also possible with a perfect optical system having no aberrations and uniform mask illumination to have variations in the size of a particular feature over the image plane, for example, if the light illuminating the mask is not properly directed into the projection lens pupil. Thus, a problem of nonuniform imaging exists.
In the past, photochromic glass has been interposed at specific locations between the light source and the mask to achieve uniform mask illumination. This is said to be achieved by the self-uniformizing action of the photochromic glass. However, the resist exposure wavelength (also referred to as the "actinic wavelength") is the same as that activating the photochromic glass. This results in an overall diminution in the beam intensity exposing the light-sensitive material of up to 50%, seriously diminishing the ability of the system to expose wafers in a timely manner. The throughput of a photolithographic system is a major driver of the overall cost per chip. Another disadvantage is that the nature of the photochromic glass self-uniformizing action precludes tailoring the beam intensity to achieve a desired end result at the wafer plane. What ultimately determines the uniformity of lithographic imaging is the uniformity of exposure at the imaging plane and not necessarily the uniformity of the illumination across the mask.
An illumination system with the flexibility to irradiate the mask in a way that creates a desired result at the imaging plane, for example, uniform imaging of a given feature over the entire wafer, would have many advantages over an illumination system only providing uniform illumination of the mask.
Thus, a need exists for an improved way to expose light-sensitive material, in particular, an improved way to expose resist on semiconductor wafers, without increasing the exposure time.